If I have a 400,000 btu boiler heating water into a 370 gallon tank and I change the tank out to a 120 gallon tank, how much capacity do I loose? Assuming 95% eff boiler.

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Round numbers, the boiler heats produces 400 gallons per hour. FIRST hour delivery is the recovery rate ( 400 gallons per hour ) PLUS 70% ot tank. SO your first hour availablilty drops from 660 gallons to 485 gallons. After the first hour, it is all recovery, 400 gallons per hour.